Micro optoelectronic elements have become of great importance in optoelectronic interconnection technologies, and the development of communications and control systems. Diffractive optical elements such as spherical, cylindrical, Fresnel lenses, aspherics and other micro-optical devices having rather precise three-dimensional profiles or contours present certain problems during mass production because of problems with product quality. In addition, the fabrication of large arrays of such optical elements covering large areas is almost always very costly.
Most chip-scale integrated optical elements, components, and functionalities are based upon waveguides that are implemented with a binary height structure. These include lenses that are microfabricated using conventional microfabrication techniques. For example, arrayed waveguide gratings (AWG) take advantage of a combination of single mode waveguides and planar multimode (or lens) regions to result in multiplexing/demultiplexing (mux/demux) functionality. In addition, micro-optical elements, such as Fresnel lenses (diffractive lenses) are often fabricated with a binary height structure. However, it is much more beneficial to the efficiency and beam quality of the lens to be able to fabricate a Fresnel lens with varying height across each zone region, or to be able to fabricate a traditional refractive lens by controlling the height of the lens with a parabolic geometry. This varying height of the feature is often referred to as gray scale fabrication.
The use of a gray scale mask fabrication process for producing large quantities or large arrays of micro-optical elements requiring high resolution of three-dimensional contours has several advantages. Gray scale masks having the desired gray scale pattern normally require only a single exposure of a photoresist when fabricating the micro-optical elements on a substrate, using an etching process. The use of gray scale masks avoids the alignment errors the frequently result from a process that requires the use of multiple binary masks. In addition, if a suitable gray scale patterned mask material is provided, thermal expansion and contraction of the masks can be substantially avoided.
A survey of prior art on using gray scale masks producing micro-optical elements shows a common dependence on the use of a photoresist layer in the fabrication of such gray scale optical structures. U.S. Pat. No. 6,071,652 and 6,420,073, and U.S. patent application Ser. No. 20020146627A1 disclose a process by which the illumination of a gray scale patterned photomask transfers the gray scale patterns into a layer of photoresist that has been deposited onto a transparent material. The pattern in the photoresist is then transferred to the transparent material into which the optical element is fabricated. The transparent material in question may be a transparent substrate or a transparent material deposited onto the substrate.
The above-discussed patents also disclose forming a micro-optical structure using gray scale technology, wherein the three-dimensionality of the structure is provided via repeated layering and development of these layers using multiple binary masks. This type of process, however, has several disadvantages. In order to fabricate, for example, curved structures using this process, many very thin layers and many different patterned binary photomasks would be necessary in order to simulate a reasonably curved structure. In addition, if relatively few layers and photomasks are utilized, the quality of the simulation of the curve would be poor due to large vertical step sizes. The former would make the fabrication process extremely slow, labor intensive, expensive, and conducive to large misalignment errors, while for the latter, in the case of a microlens for example, the lensing efficiency would be dramatically decreased.
Another technique for forming a refractive micro-optical element includes forming a structure in a photoresist by patterning and melting a photoresist layer on a glass substrate. The melting of the photoresist generates spherical surfaces. This technique is limited to special shapes and can only provide spherical contours using a thin positive photoresist layer. The refractive element is produced by ion milling the photoresist structure and then the glass substrate. The ions first mill the photoresist and, after the photoresist is removed in a region, the glass substrate is milled. In this manner the photoresist structure is transferred to the glass substrate and the refractive micro-optical element is formed.
In U.S. Pat. No. 6,301,051, a process is disclosed wherein a planarized material is deposited onto an opto-electronic device substrate and the material is coated with a layer of a photoresist. Using a gray scale patterned photomask, a thickness contour matching the photomask gray scale is then patterned into the photoresist photolithographically. The contour pattern is then transferred to the planarized material by ion etching.
The prior art processes previously discussed have in common the required inclusion of a photoresist in the fabrication process. Among the characteristics that differentiate the present method of fabricating a micro-optical element from the prior art processes is that, in the subject method, the negative photosensitivity of the material in which the micro-optical elements are fabricated allows the exclusion of the photoresist material from the process. The claimed method using a spin-on glass material (a sol-gel) and allows the gray scale fabrication of a glass-like material (a spin-on glass material or SOG material) having added vertical functionality and design possibilities because the structures can be planarized with varying heights (as desired) after each coating (preferably spin-coating) layer.